A technique by which transistors are formed using semiconductor thin films over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a transistor whose active layer includes an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration lower than 1018/cm3 is disclosed (see Patent Document 1).
A transistor including an oxide semiconductor is known to have a problem of low reliability because of high possibility of change in electrical characteristics, although the transistor including an oxide semiconductor can operate at higher speed than a transistor including amorphous silicon and can be manufactured more easily than a transistor including polycrystalline silicon. For example, comparing the transistor before a BT test under light irradiation with that after the BT test under light irradiation, the threshold voltage of the transistor is changed by 1 V or more.